An equalizer is typically used in receivers to reduce noise and communication channel effects which would otherwise make the accurate recovery of transmitted data difficult. Equalizers are frequently implemented as time domain filters which multiply received signals by a plurality of taps and which sum the multiplication results to produce corresponding equalizer outputs. The values of an equalizer's taps are adjusted during operation of the equalizer to optimize the equalizer's performance. Feed forward or feedback control may be used for the tap value adjustment.
In a feed forward approach, a channel estimator may be used to estimate the channel, and the values of the equalizer's taps are modified based upon the estimated channel. In a feedback approach, an output of the equalizer is compared either with a training signal or with decoded data, although both may be used. If both a training signal and decoded data are used, the training signal may be used at start up because the initial values of the equalizer's taps may not otherwise permit the equalizer to converge on its optimized tap values. Once convergence is achieved, decoded data may be used to maintain convergence. That is, assuming that the equalizer is operating optimally, decoded data downstream of the equalizer should be the transmitted data and can be used as feedback in order to control the values of the equalizer's taps.
During periods of time varying noise conditions within the communication channel, however, the equalizer can lose its convergence. During such periods, stabilization of the equalizer's behavior can be attempted by permitting the equalizer to coast until the noise condition has dissipated. As the equalizer coasts, however, the equalizer can drift away from its optimized tap values.
The present invention is directed to an arrangement which effectively controls the tap values of an equalizer even during periods of time varying noise conditions.